Electro-neutral KCl co-transporters have been described in a number of different cell types and play important roles in cell volume regulation and in trans-epithelial chloride transport in many nephron segments. The study of KCl co-transporters was limited by a lack of molecular information. Recently the first KCl co-transporter was cloned. Subsequently the principal investigator has cloned two completely new co-transporters KCC3 and KCC4. The long-term goals of this grant proposal are to clarify the molecular mechanisms and physiological role of these KCl co-transporters. Using the newly completed protein sequences the principal investigator has designed a panel of specific antibodies. The goal is to use these antibodies to study the distribution and regulation of the three renal KCCs. Functional studies of these co-transporters will determine which intracellular amino acids determine the volume sensitivity of KCC3 and KCC4 and which transmembrane amino acids determine cation affinity.